1. Field of the Invention
The present invention relates to a charging member, a charging device, an image forming apparatus, and a process cartridge.
Here, an image forming apparatus refers to an apparatus that forms an image on a recording medium by an electrophotographic image forming method. Examples of image forming apparatuses are an electrophotographic copying machine, an electrophotographic printer (e.g., a laser beam printer or an LED printer), a facsimile machine, and a word processor.
A process cartridge refers to a cartridge serving as a process unit into which at least a charging device and an electrophotographic photosensitive member are combined, and which is removably mounted in a main body of the image forming apparatus.
2. Description of the Related Art
(1) Image Forming Process
FIG. 2 schematically shows a configuration of an image forming apparatus of the related art. This image forming apparatus is an electrophotographic copying machine, printer, facsimile machine, or word processor. An electrophotographic photosensitive member 100 shaped like a rotating drum (hereinafter referred to as a photosensitive drum) is rotated at a predetermined peripheral speed in a clockwise direction shown by the arrow. During rotation, the photosensitive drum 100 is uniformly charged to a predetermined polarity and potential by a charging device 101, and is then subjected to image exposure by an exposure device 102, whereby an electrostatic latent image is formed on a surface of the photosensitive drum 100. The electrostatic latent image is developed into a visual toner image by a developing device 103. The toner image on the surface of the photosensitive drum 100 is transferred, by a transfer device 105, onto a recording medium 104, such as paper, supplied from a sheet feeding section (not shown). The recording medium 104 on which the toner image is transferred is separated from the surface of the photosensitive drum 100, and is guided into a fixing device 106, where the toner image is fixed. After that, the recording medium 104 is ejected as an image-bearing medium. After the recording medium 104 is separated, the surface of the photosensitive drum 100 is cleaned with a cleaning device 107 by scraping off residual toner, and is repeatedly used for image formation.
(2) Charging Device
A charging bias source applies a charging bias voltage to a charging member of the charging device 101. In a typical charging method for applying only a direct-current voltage as a charging bias voltage, discharging occurs when a voltage more than or equal to a certain threshold voltage is applied, and this charges the photosensitive drum 100 (hereinafter this charging method is referred to as DC charging).
U.S. Pat. No. 4,851,960 discloses a charging method for applying a bias voltage obtained by superimposing, on a direct-current voltage Vdc corresponding to a desired dark potential Vd on the drum, an alternating-current voltage having a peak-to-peak voltage Vpp that is more than or equal to double that of a discharging start voltage at the application of the direct-current voltage. Hereinafter, a direct current is referred to as a DC, an alternating current is referred to as an AC, and this charging method is referred to as AC/DC charging. This charging method is excellent in uniformly charging the photosensitive drum 100. When an AC voltage higher than or equal to a predetermined voltage is superimposed on a DC voltage, local unevenness in potential (charging failure) on the photosensitive drum 100 is overcome by a potential uniforming effect of an AC component, and the charged potential Vd on the surface of the photosensitive drum 100 uniformly converges to the DC voltage Vdc. In AC/DC charging, however, the value of discharging current for the photosensitive drum 100 is larger than in DC charging for applying only a DC voltage. For this reason, chains linking molecules on the surface of the photosensitive drum 100 are easily cut, and the photosensitive drum 100 is easily shaved by a cleaning blade of the cleaning device 107. This shortens the life of the photosensitive drum 100.
The charging device 101 typically adopts a contact charging method that charges the surface of the photosensitive drum by applying a voltage to a charging member that is shaped like, for example, a roller or a blade and that is in contact with the surface of the photosensitive drum. In particular, a charging method using a roller allows stable charging over a long period of time.
However, the charging roller is soiled with a soiling substance through repetitive image forming processes, and nonuniform charging resulting from the soiled charging member sometimes causes image failure such as unevenness in image density and scumming. Soiling of the charging member is caused by adhesion of part of the toner, which remains on the photosensitive drum 100 after transfer, onto the charging roller. To overcome this problem, Japanese Patent Laid-Open Nos. 2007-298820 and 2008-122781 disclose a technique of reducing adhesiveness of a soiling substance onto a charging roller by decreasing the surface roughness Rzjis of the charging roller. This technique has a certain effect on soiling of the charging member.
Japanese Patent Laid-Open No. 3-101768 discloses a technique of reducing soiling of a charging roller by sliding a cleaning member on the charging roller. Further, Japanese Patent Laid-Open No. 10-213945 discloses another technique that is effective against soiling of a charging member. In this technique, a cleaning member for a charging roller functions as a charge application member, and applies charge to toner serving as a soiling substance so as to move the toner onto a photosensitive drum.
A description will now be given of the background art of the problems to be solved by the present invention.
It is known that a soiling substance formed by microparticles (hereinafter referred to as soiling microparticles), such as part of toner remaining on a photosensitive drum after transfer, soils a charging roller. Adhesion of soiling microparticles causes local unevenness in potential on the photosensitive drum, and this is one of the factors that cause image failure such as nonuniform density and scumming.
A description will now be given of an example of a process in which soiling microparticles are produced.
For example, in an image forming apparatus equipped with a cleaning member, when part of toner remaining on a photosensitive drum after transfer passes by the cleaning member, it sometimes adheres onto a surface of a charging roller in contact with the photosensitive drum. Further, in a cleaner-less method for removing toner, which remains on a photosensitive drum after transfer, by cleaning performed simultaneously with development and recovering the toner for reuse, more soiling microparticles exist on the photosensitive drum, and soil the charging roller. When microparticles other than the toner, for example, external additives, paper dust, shavings of the photosensitive drum, microparticles floating in the air, and microparticles adhering to paper, adhere onto the charging roller, nonuniform charging also occurs. For this reason, there has been a demand for a charging member onto which soiling microparticles do not easy adhere.
Particularly in DC charging, the photosensitive drum is less susceptible to shaving and has a longer life than in AC/DC charging. On the other hand, since a uniforming effect of an AC component is not provided, unevenness in potential is easily caused on the photosensitive drum by soiling of the charging roller, and image failure easily occurs.
When the charging roller is driven by rotation of the photosensitive drum, it is known that the charging roller achieves a pronounced effect in scraping the soiling substance off the photosensitive drum.
In addition, in the image forming apparatus using the cleaner-less method, the charging roller is exposed to more soiling substances.